Magnetism and bearings – MRO MagazineMRO Magazine

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The issue of allowable magnetism in bearings arose as a result of magnetization of a shaft when a manufacturing company cut off a part of its shaft on which the bearing was to be mounted. Since the bearing had to be mounted on this magnetized shaft, it was estimated that the shaft would also make the bearing magnetized. It would be like this experiment that a child does with a magnet and paper clips, to see how long a string of paper clips could be made from the magnet.

What’s wrong with a bearing being magnetic?
Iron is one of the most abundant elements on earth, and iron oxide is a great way to grind it. If you have a precision magnetic machine that attracts iron / iron oxides, you generate a situation where a bearing pulls that grinding medium towards it, and it can grind itself. In addition, as the bearing wears out, the resulting product is itself a grinding medium multiplying the problem. Having a bearing that attracts a grinding medium is not a good thing.

Photo credit: Maurer Magnetic AG.

There is not a lot of material published on this topic; however, there is an article published by the Electrical Apparatus Service Association (EASA) – “Demagnetizing Motor Shafts to Prevent Bearing Failures” by Cyndi Nyberg – in the October 2005 issue of Currents. He suggests that a measurement of no more than two gauss should be taken in the bearing area of ​​the shaft.

For a typical maintenance shop (as opposed to an electric motor shop), such an instrument for measuring gauss is not common. However, it is clearly recognized that regardless of the manufacturing processes of a shaft that a bearing will be mounted on, there must be a way to measure and remove the magnetism that can be created by this process.

Photo credit: Maurer Magnetic AG.

Another cause of magnetism is induction heating, a common tool in stores that install bearings. It creates an electric field that heats the bearing (like a microwave oven), but it also causes magnetism in the bearing. More sophisticated induction heaters have an automatic demagnetization cycle (demagnetization cycle); however, with less expensive systems, the bearing must be “stirred through the uprights” while the induction heater is on, to ensure proper demagnetization. This is of concern, as it depends on the transmission of knowledge, as this type of induction heating lacks specific instructions to alert the user to this key step in the bearing heating process.

For induction heaters with demagnetization cycles, the stated “norm” is that the magnetism should be less than two amps / centimeter (a / cm) (of interest, one a / cm, is about 0, 000126 tesla and one tesla is 10,000 gauss). These premium induction heaters ensure that they do not leave the bearing with magnetism and thus unintentionally shorten its service life.

Another common case of magnetism in a bearing is the failure of a bearing. The bearing must be removed from the shaft or housing, and it is cut with a zip cutter or other grinding / cutting device. This cutting action will impart magnetism to the bearing rings.

Later, when the bearing is inspected by a technician, they may notice that this bearing is magnetic. Is this what caused the failure? No, just like in the first case, cutting a ferrous metal can induce magnetism. The inspected bearing is magnetized, but this was caused by the removal process.

Bearing companies understand the processes by which magnetism can be induced and apply the appropriate technology to correct this problem. A company in Switzerland supplies equipment to bearing manufacturers to properly demagnetize bearings and components in the manufacturing process.

One of the bearing manufacturing processes identified as a cause of magnetization is the hot riveting of a brass cage in a cylindrical roller bearing. This process causes the assembled bearing to become magnetic. As a rule, this is corrected by the appropriate tools in the manufacturing process.

Having a bearing that has become magnetic can be detrimental to its operation. The sources of magnetism in the bearing manufacturing process are well understood and tightly controlled. Therefore, the likelihood of a new bearing being magnetic from the packaging is highly unlikely. However, further manipulation of the bearing, such as using induction heating without a demagnetization cycle, or having the shaft itself become magnetic from its manufacturing process, can induce damaging magnetism in the bearing. the bearing.

Is there such a thing as a “Magnetic bearing”?
Yes there is. “Magnetic bearings” are used in applications such as high speed compressors for natural gas installations and compressors for chillers in large urban buildings. The shaft is supported by an electromagnetic field, which enables oil-free operation and speeds up to 40,000 rpm. Due to the complexity of these bearings, which levitate the shaft, these bearings are expensive and uncommon. MRO


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